The present invention relates to a modem signal detecting system for an adaptive differential PCM CODEC providing a coding characteristic suitable for modem signals by detecting the start and the end of modem communication.
An adaptive differential pulse code modulation coding system (herein-after referred to as ADPCM coding system) is intended to effectively transmit voice signals with lower transmission rate (bandwidth) as compared with the conventional PCM system by utilizing the statistic characteristic of the power spectrum of voice signals shown in FIG. 1.
In application of such ADPCM coding system in an existing communication network, it is desirable to provide data transmission of a voice band modem signal in addition to the voice signal, but the power spectrum of the voice band modem signal shows less fluctuation, as shown in FIG. 2, apparently indicating different statistic characteristic, while the signal power of the voice signal shows fluctuation of about 40 dB.
For the ADPCM coding system, the CCITT recommends (G721) a coding algorithm for realizing the voice signal transmission with the bit rate of 32 kbits/sec by reducing the bandwidth of the voice signal to 1/2. This coding algorithm utilizes, as explained previously, the statistic characteristic of the power spectrum of the voice signal shown in FIG. 1. Accordingly, such algorithm is not suitable for coding the modem signal having the power spectrum as shown in FIG. 2 and it only compensates for the coding of the modem signal with a maximum bit rate of 4,800 bit/sec.
Therefore, the modem signal of 9,600 bit/sec which is most widely used under the regulation GIII for the facsimile communication cannot be transmitted with this coding algorithm.
To avoid the above disadvantage, a system has been proposed in U.S. Pat. No. 4,788,692, the disclosure of which is incorporated herein by reference, which system switches the coding algorithm to the algorithm for the modem communication only when the modem communication is decided by detecting a modem training signal. The modem training signal is detected in this system by monitoring the initial modem training signal of 9600 bit/sec. Namely, when an input signal drops to a value exceeding the decided level, the end of the modem communication is detected. Such modem communication should be always detected even in case a transmission line noise level is comparatively high.